1. Field of the Invention
This invention relates to a method of making a group III nitride-based compound semiconductor. This method is useful for the fabrication of various semiconductor devices, such as a semiconductor light emitting element and a semiconductor light receiving element.
2. Description of the Related Art
Conventionally, a technique is known that so called off angle (off-orientation) is introduced to the crystal growth surface of a crystal growth substrate so as to grow a high-quality semiconductor crystal. See, for example, Japanese patent application laid-open Nos. 2000-174395 (prior art 1), 2000-156348 (prior art 2), 2001-160539 (prior art 3) 2002-16000 (prior art 4) and 2003-60318 (prior art 5), and Tsunenobu KIMOTO and Hiroyuki MATSUNAMI, “Step-controlled epitaxy in semiconductor SiC polytypes”, OYO BUTURI, vol. 64, No. 7, pp. 691-694 (1995) (prior art 6).
The typical or general function and principle of the step-flow growth induced by the setting of off-angle is described in prior art 6.
Prior arts 1 and 2 describe about trying to grow a high-quality group III nitride-based compound semiconductor on a heterosubstrate, such as sapphire, ZnO and spinel, of a different kind from the targeted growth layer (group III nitride-based compound semiconductor).
Prior arts 3 to 5 state that an off angle of 1 degree or more is desirable.
In growing a group III nitride-based compound semiconductor on a heterosubstrate, the matching of lattice constant on crystal growth surface is critical. Therefore, it is necessary to grow a buffer layer thereon to obtain a high-quality growth layer (semiconductor crystal). Thus, the method of using the heterosubstrate is not always desirable in terms of the quality and the number of fabrication steps concerning the targeted group III nitride-based compound semiconductor.
FIG. 1 is a schematic perspective view illustrating the problems of conventional techniques. As shown in FIG. 5, if the off angle is set to be more than 1 degree, the problem of step amplification occurs. For example, when a c-face of crystal growth substrate 1 of group III nitride-based compound semiconductor (e.g., GaN) is polished while having the off angle, a step portion y of about 1 atom layer to several tens of atom layers is formed on the polished surface σ. Subsequently, when a growth layer 2 of group III nitride-based compound semiconductor (e.g., GaN) is grown on the polished surface a with the step portion y, a giant step Y that the height of step portion y is amplified may be generated on the upper side of growth layer 2. In this case, the width of a terrace (the interval of parallel giant steps Y) can be also significantly greater than that of the atom-layer height-formed step portion y on the polished surface σ.
The height of giant step Y is likely to increase according as the off angle of crystal growth substrate 1 increases. If the height of giant step Y becomes too high, the flatness of growth layer 2 (targeted group III nitride-based compound semiconductor) obtained thereupon will deteriorate. When using it for high-quality semiconductor devices, unignorable problems in terms of driving efficiency (current, voltage), operation life, heat generation etc.
On the other hand, if the off angle of crystal growth substrate 1 is too small, the growth layer 2 cannot grow based on the step-flow growth and, therefore, it is impossible to obtain a semiconductor crystal that has a good crystalline quality and a sufficiently flat surface. This can be proved by the general function and principle of step-flow growth as described in prior art 6.